PL/Tcl has support for autoloading Tcl code when used. It
recognizes a special table, pltcl_modules, which is presumed to contain
modules of Tcl code. If this table exists, the module unknown is fetched from the table and loaded into
the Tcl interpreter immediately before the first execution of a
PL/Tcl function in a database session. (This happens separately
for each Tcl interpreter, if more than one is used in a session;
see Section 39.4.)

While the unknown module could
actually contain any initialization script you need, it normally
defines a Tcl unknown procedure
that is invoked whenever Tcl does not recognize an invoked
procedure name. PL/Tcl's
standard version of this procedure tries to find a module in
pltcl_modules that will define the
required procedure. If one is found, it is loaded into the
interpreter, and then execution is allowed to proceed with the
originally attempted procedure call. A secondary table pltcl_modfuncs provides an index of which
functions are defined by which modules, so that the lookup is
reasonably quick.

The PostgreSQL distribution
includes support scripts to maintain these tables: pltcl_loadmod, pltcl_listmod, pltcl_delmod, as well as source for the standard
unknown module in share/unknown.pltcl. This module must be loaded
into each database initially to support the autoloading
mechanism.

The tables pltcl_modules and
pltcl_modfuncs must be readable by all,
but it is wise to make them owned and writable only by the
database administrator. As a security precaution, PL/Tcl will
ignore pltcl_modules (and thus, not
attempt to load the unknown module)
unless it is owned by a superuser. But update privileges on this
table can be granted to other users, if you trust them
sufficiently.